Hi
@leamason and welcome to the forums.
I don't know about the insurance aspect (I'm in New Zealand which has a public health system) but have a few thoughts which may or may not help.
Can you see a different doctor? Has he seen your records which show high blood sugars when unmedicated?
While 300mg/dL is high (in UK units that is 300/18 = 16.7mmol/L) many T2s go much longer than 3 months with higher levels. The coma risk is if that is combined with high ketones because you aren't making insulin. (I totally agree that you probably won't feel well and it seems crazy that you are being denied the medication that you need because of a technicality).
However, could you make an appointment for much sooner than 3 months so that the doctor can see your high readings?
Good luck.